Depending on the magnitude and type of risk, pipe lines subject to corrosion risks are protected by a passive or active protection system or a combination of both. The passive protection is comprised of a coating, the active protection is either a cathodic protection or a protection via a so-called sacrificial anode.
The coating must be electrically insulating in order to avoid direct contact of the pipe line with the electrically conducting medium ‘ground’ or ‘water’ and therewith a corrosive attack on the pipe line.
In the case of cathodic corrosion protection of underground pipe lines provided with an electrically insulating coating the pipe line is protected against corrosion at surface defects of the insulating encasing thereby that to the pipe line a DC voltage is so connected that the pipe line has an electrically negative potential with respect to the surrounding soil, or acts as a cathode. For this purpose the negative pole of a DC voltage source is connected to the pipe line and its positive pole, or the anode, is grounded.
The active protection can alternatively also take place by way of applying so-called sacrificial anodes to the pipe line. The basic principles are described, for example, in the Handbuch des kathodischen Korrosionsschutzes, VCH Verlag, Third Edition 1989.
The effectiveness of the passive protection of the pipe line via the coating is a function of the quality, i.e. the freedom from defects, of the coating. Damages on the coating, which cause an electric contact of the pipe with the soil or the water, lead to a corrosive attack on steel pipes to be protected, which can result in large-area delaminations of the coating. The extent of delamination depends herein on the type of coating system and the obtaining corrosion conditions (for example conductivity of the ground, pH-value, etc.).
Damage to the coating can occur, for example, during the transport of the pipe such that before laying the pipe the coating is customarily inspected for defects and the sites of the defect are repaired.
However, the coating can also be damaged during the operation of the pipe line, for example during earth work carried out in the proximity of a laid pipe line, through microbiological attacks or settlements and shifts in the ground.
This requires that the pipe line is regularly checked for potential damage of the coating in order to avoid greater corrosion damage, which is especially important in the case of pipe lines carrying oil or gas, in order to prevent leakages due to corrosion damage.
Relatively frequent line inspections are, however, not acceptable for economic reasons since the measurements are complex and expensive.
Methods for locating defects known in prior art are described, for example in Handbuch des kathodischen Korrosionsschutzes, VCH Verlag, Third Edition, 1989, pp. 112-124. Conventionally, in this method from the trace-form of the protective current and the potential in defined line sections conclusions are drawn regarding possible damage in the coating.
A further method based on measurements of the shape of the potential and the determination of the resistance of the coating over a modulated cathodic protective current is disclosed in DE 690 14 002 T2.
Of disadvantage in these known methods is, however, that, for example in the pipe line sections to be examined, signals from current-carrying lines crossing the pipe line are also acquired and the acquired signal indications can no longer be uniquely assigned to a possible coating defect. The reason is that the analyzed current trace is composed of the fed protective current and the external current signal.
Of disadvantage is, furthermore, that these methods can only be applied in combination with a cathodic protection of the pipe line.
EP 0560 443 B1 discloses a method for monitoring and localizing damages on the coating of a cathodically protected underground pipe line, in which by applying superimposed local sinusoidal exciting currents of different frequencies to the pipe line and measuring the corresponding voltage responses or the impedance, possible damages are detected. Herein with a mobile measuring system the pipe line is examined in constant 2 meter long sections for possible damages of the coating.
This method has several disadvantages. For one, the method can also only be applied in combination with a cathodic protective device. Furthermore, it cannot be excluded that via external effects interference signals are also acquired and therewith a unique defect assignment is no longer possible. Furthermore, it is disadvantageous that for the application of this method an examination directly on the pipe is required, which is highly time-consuming and expensive in long pipe lines to be tested.